OK, henceforth all new users are MODERATED. This is how I had things set up on Yahoogroups. Up until now, the captcha plus the question authentication was enough to thwart these guys along with, get this, the 15,000 PLUS banned ip addreses of known spammers. (see for yourself at
Stop Forum Spam - Forum Banlist)
Being moderated is an inconvenience for legitimate new users but keeps the trash from reaching all the others. When a new user registers I will see any new messages they post before they go to the world. If it is spam, user is banned and message deleted. If it is a legotimate message they will be set to unmoderated.
By now, any former Yahoogroup users should have moved over so it is a small price to pay.